Wireless microphone systems configured to transmit voice via radio waves require no cable to connect a microphone handset and a base unit (microphone amplifier) together, so that they can be used flexibly in various use environments. For this reason, such wireless microphone systems are used in various places such as public and private facilities, various school facilities, and stores, for example. Of these wireless microphone systems, there is known a wireless microphone system configured to transmit digital voice data from a microphone to a base unit. In a case where a plurality of radio communication systems including this wireless microphone system exist within the same area, transmission of radio waves from a base unit to a microphone handset using maximum transmission power always enables communication even when the base unit and microphone handset are distant from each other. In this case, however, there is a problem in that the transmission of radio waves using the maximum transmission power always causes an increase in interference to other radio communication systems.
Given this, there have been developed radio communication systems configured to variably control the transmission power of the base units. For example, Japanese Patent Application Laid-Open No. 2001-332987 (hereinafter, referred to as “PTL 1”) discloses a technique that sets the optimum transmission power by controlling transmission power such that the transmission power value of a radio communication apparatus is made the maximum value at the start of communication, then reduced by a predetermined amount each time transmission succeeds, and raised by a predetermined amount when transmission fails, for example.
Japanese Patent Application Laid-Open No. 2002-345026 (hereinafter, referred to as “PTL 2”) discloses a technique that controls the transmission power of the base unit (connection apparatus) according to whether or not a handset is linked to a charging cradle. That is, PTL 2 discloses the technique that performs communication with a low transmission power when the handset is linked to the charging cradle and that performs communication with a high transmission power when the handset is removed from the charging cradle. Thus, the technique disclosed in PTL 2 makes it possible to sustain the communication between a handset and a connection apparatus even if an interfering signal is received while the handset moves away from the connection apparatus.